This invention relates to B-stageable underfill compositions suitable for application to a silicon wafer before singulation. The compositions contain two separately curing chemistries.
Microelectronic devices contain millions of electrical circuit components that are electrically connected to each other and electrically connected to and mechanically supported on a carrier or a substrate. The connections are made between electrical terminations on the electronic component and corresponding electrical terminations on the substrate.
One method for making these interconnections uses polymeric or metallic material that is applied in bumps to the component or substrate terminals. The terminals are aligned and contacted together and the resulting assembly is heated to reflow the metallic or polymeric material and solidify the connection.
During its normal service life, the electronic assembly is subjected to cycles of elevated and lowered temperatures. Due to the differences in the coefficient of thermal expansion for the electronic component, the interconnect material, and the substrate, this thermal cycling can stress the components of the assembly and cause it to fail. To prevent failure, the gap between the component and the substrate is filled with a polymeric encapsulant, hereinafter called underfill or underfill encapsulant, to reinforce the interconnect material and to absorb some of the stress of the thermal cycling.
Two prominent uses for underfill technology are for reinforcing packages known in the industry as chip scale packages (CSP), in which a chip package is attached to a substrate and flip-chip packages in which a chip is attached by an array of interconnections to a substrate.
In conventional underfill applications, the underfill dispensing and curing take place after the reflow of the metallic or polymeric interconnect. If the interconnect is a metal solder composition, a fluxing agent initially is applied on the metal terminal pads on the substrate. The semiconductor chip is placed on the fluxed area of the soldering site. The assembly is then heated to allow for reflow of the solder joint, or reflow of the polymeric interconnect. Next, a measured amount of underfill is dispensed along one or more peripheral sides of the electronic assembly and capillary action within the component-to-substrate gap draws the material inward. After the gap is filled, additional underfill encapsulant may be dispensed along the complete assembly periphery to help reduce stress concentrations and prolong the fatigue life of the assembled structure. The underfill encapsulant is subsequently cured to reach its optimized final properties.
In another conventional method, the underfill is dispensed onto the substrate. A bumped chip is placed active-face down on the underfill and the assembly heated to establish the solder or polymeric interconnections and cure the underfill.
Recently, attempts have been made to streamline the process and increase efficiency by placing the underfill encapsulant directly onto the semiconductor wafer before it is diced into individual chips. This procedure, which can be performed via various methods, including screen printing, stencil printing and spin coating, allows for a single application of underfill to a semiconductor wafer, which is later diced into multiple individual chips.
In order to be useful as a wafer level underfill encapsulant, the underfill must have several properties. The material must be easy to apply uniformly on the wafer so that the entire wafer has a consistent coating. During the final attachment of the individual chips to a substrate, the underfill must flow to enable fillet formation, flux the solder bumps if solder was used and provide good adhesion. Whether the interconnection of the chip to the substrate is made with solder or with polymeric material, curing of the underill should occur after the interconnection is formed and should occur rapidly.
Another important property is that the underfill must be able to be solidified after application to the wafer so as not to interfere with the clean dicing of the wafer into individual chips. The solidification of the underfill encapsulant is done by a process called B-staging, which means that the underfill material undergoes an initial heating after its placement on the wafer to result in a smooth, non-tacky coating without residual solvent.
If the starting underfill material is a solid, the solid is dispersed or dissolved in a solvent to form a paste and the paste applied to the wafer. The underfill is then heated to evaporate the solvent, leaving a solid, but uncured, underfill on the wafer. If the starting underfill material is a liquid or paste, the underfill is dispensed onto the wafer and heated to partially cure it to a solid state.
The B-stage heating typically occurs at a temperature lower than 150xc2x0 C., preferably within the range of about 100xc2x0 C. to about 150xc2x0 C. The final curing of the underfill encapsulant must be delayed until after solder fluxing (when solder is the interconnect material) and the forming of the interconnection, which occurs at a temperature of 183xc2x0 C. in the case of tin-lead eutectic solder.
This invention is an underfill composition comprising two chemical compositions have curing temperatures or curing temperature ranges sufficiently separated to allow the composition with the lower curing temperature, hereinafter the first composition, to cure without curing the composition with the higher curing temperature, hereinafter the second composition. In practice, the first composition will be cured during a B-staging process, and the second composition will be left uncured until a final cure is desired, such as, at the final attach of a semiconductor chip to a substrate. The fully cured material is cross-linked or polymerized to a sufficiently high molecular weight effective to give it structural integrity.